1. Field of the Invention
This invention relates to a semiconductor sensor for detecting a physical quantity such as acceleration, pressure, vibration, or angular speed, and to a method of manufacturing the semiconductor sensor.
2. Description of the Related Art
JP-A-9-211022 disclosed this kind of semiconductor physical quantity sensor formed through steps of forming an SOI substrate from two silicon substrates by bonding, forming trenches in the SOI substrate at an upper silicon substrate side, and carrying out sacrifice layer etching to form a beam structure with a movable electrode. In this method using the SOI substrate, however, it is necessary to additionally perform steps of forming a wiring pattern on a lower side silicon substrate before bonding, flattening a surface of one of the substrates for the bonding, polishing the upper side silicon substrate after the bonding, and the like, resulting in complicated processes.
On the other hand, JP-A-6-123631 discloses a method of manufacturing an angular speed sensor having a movable portion. Specifically, a wafer composed of a silicon substrate on which an N-type epitaxial layer is formed and in which a P-type buried layer is formed is prepared. Then, trenches are formed in the N-type epitaxial layer with a specific pattern, and the P-type buried layer is removed by electrochemical etching through the trenches so that the movable portion is formed. In this method, because the sensor dispenses with an SOI substrate, the manufacturing process is simplified. However, the wafer composed of the silicon substrate on which the N-type epitaxial layer is formed and in which the P-type buried layer is formed must be utilized.
When this kind of semiconductor physical quantity sensor is formed by electrochemical etching, a wafer composed of a P-type silicon substrate on which the N-type epitaxial layer is formed can be used. For example, when a semiconductor pressure sensor is manufactured from the wafer, a diaphragm for the pressure sensor can be formed by electrochemically etching the wafer from a wafer back surface.
It is also possible to manufacture the acceleration sensor having the same structure as that disclosed in JP-A-9-211022 from the wafer including the P-type silicon substrate on which the N-type epitaxial layer is formed. Specifically, after trenches are formed in the wafer on a front surface side, electrochemical etching is carried out on the wafer from the front surface side so that the movable portion of the sensor is formed. At that time, however, the N-type epitaxial layer is not etched and the P-type silicon substrate is etched. Therefore, it is difficult to precisely control etching conditions such as etching time for accurately forming the movable portion.
In addition, when the electrochemical etching step is carried out on the wafer, the wafer is immersed into an anisotropic etching solution, and a positive voltage is applied to the N-type epitaxial layer so that only the P-type silicon substrate is etched. Therefore, it is necessary to form wiring segments within respective chips for applying the positive voltage to the N-type epitaxial layer during the electrochemical etching step. This makes an area of each chip large.